Abstract: A process for the thermal cleavage of 1,2-dichloroethane to give vinyl chloride at 300.degree. to 600.degree. C. under atmospheric pressure or elevated pressure is described. Before cleavage of the 1,2-dichloroethane, 0.001 to 5% by weight of trichloroacetyl chloride or a compound which contains 3 carbon atoms, at least 6 chlorine atoms, 0 to 1 oxygen atom and, for each carbon atom bonded to the latter, 0 or 1 hydrogen atom, is added to it. Due to the addition of the compound(s) mentioned, the conversion is increased at the same cleavage temperature, or the cleavage temperature is reduced at the same conversion, at the same time a reduction in interfering byproducts being observed.